Workshop on Light

Light is a fundamental concept in physics and it is not a surprise, as vision is one of our most powerful sensory experience. Studying light is an opportunity for teachers to enthuse students about science. One does not need a sophisticated laboratory setup with costly equipments to understand the laws of nature; nature is in itself a laboratory. All we need is the ability to help students relate it to observations that they can make and think about. If we can understand the phenomenon behind a rainbow in its entirety, then we have understood the phenomena of light.

A workshop was organized for teachers where Dr. Alok Sharan, Professor from the Pondicherry University deliberated on the topic of light. The essence of his session was that how simple observations can be powerful tools in constructing the concepts. Below is a summary of the discussion.

Source of Light

Our understanding of light has significantly changed from ancient times. In the Greek era, Aristotle believed ‘the eye’ to be the source of light.  He concluded it based on the observation that we cannot see anything when we close our eyes. In the scientific world, any hypothesis holds true as long as it cannot be falsified. What then are scenarios where this is not true…One such instance is that, if the eye was the source of light, why are we not able to see at night? Investigating this thought further led us to disprove that eye is not the source of light. This led way for new answers to come in and new questions to spring up. Thus, our understanding about light started to evolve.

Our understanding of light and its nature has been challenged and modified through the ages. This shows an important aspect about the “scientific method” - no knowledge is regarded as the ultimate truth. New observations and insights can always modify or replace existing theories/hypothesis. Students in our classrooms may also come up with various hypothesis. It is important to direct children to be able to test their hypothesis and validate it.

Different phenomenon of light

Reflection

From dressing table mirrors to large telescopes, the property of reflection has many real-life applications. What is reflection? It is the bouncing back of a beam of light when it strikes a surface. However, we see an image only when it strikes a smooth surface and not on rough surfaces. Why is this so? To understand this, we must first understand how light travels in a medium. Light travels in a straight line. How do we know this? The clue lies in the formation of shadows! Shadows are formed because the object blocks the path of light and if light could bend around the object (which they can in some situations that are not found on earth) then there would be no shadows formed.

fig.1 Reflection from a smooth and irregular surface

Why can’t we see our image on the wall where as we see our reflection on a smooth window pane/surface?

To answer this, we must know how an image is formed. An image is formed by one to one correlation of the light rays that forms the image. Every point on the object should be mapped to only one point on the screen where the image is formed, it is like one to one mapping as we say in mathematics. So, when the light rays are reflected off a rough surface the one to one correlation is lost and hence the image is not formed, but whereas on a smooth surface the correlation is not disrupted and this gives us the image. Fig1.

Refraction

Why does the size of the lemon appear to be larger inside a glass filled with water? During hot weathers when we travel on a highway we see small puddles of water from a distance but when we go near to it they disappear, how?

This happens due to refraction, which is nothing but the bending of light, as it moves from one medium to another. Newton believed light consists of tiny particles that move in a straight line. He explained reflection using this theory - light consisted of small spherical balls that bounced back when hit on a surface. However, he was unable to explain refraction using this theory. Why? Because light is not formed of particles but is also a form of energy. Why does light choose to bend when it enters another medium? Yes, so light is a form of energy and it does not gain or lose energy during propagation, hence the light tends to take a shorter path to conserve its energy, which is why it bends in a medium to leave it as soon as possible. This is explained better using Ferrets law, which states that light, takes the least path to travel from one point to another, so in order to reduce its time; it takes a shorter distance through the medium.

So why does the lemon look enlarged? It is because there are three mediums through which the light has to pass - water, glass and air (fig 2).  Therefore, there is a displacement of the incident ray when it passes through these materials, which causes the size of the lemon to appear larger.

fig 2. Light passing through air, glass and water

Why does the mirage appear only on hot days? As we all know land gets heated up during summer and the subsequent air above the surface of the land/road is hotter than the layers above it. So naturally, a temperature gradient is generated. Because of this, the refractive index of the atmosphere varies with the distance as we move away from the surface of the earth as shown in fig 3.

fig 3. Mirage Formation

As you see from fig 2. & fig. 3 any physical phenomenon of light can be represented in the form of ray diagrams. However, a few things need to be kept in mind. The rays that we draw must be in accordance with the laws of reflection; angle of incidence is equal to the angle of reflection. Care must be taken while drawing the normal; for flat surfaces, the normal will be perpendicular to the surface but for curved surfaces like mirrors or lenses, the normal must be drawn after drawing a tangent as shown in the fig 4.

fig. 4 Drawing normal for curved surfaces

We usually only depict two rays of light coming from the object, which results in the image formation. What happens to the rest of the rays? The two rays that we draw are the boundary conditions for forming the image. Any other ray, which is outside the purview of the two rays, will not be able to form the image on the screen. Therefore, it is not included in the ray diagram.

Our understanding about light has evolved over a long period, with observations and experiments. Our pursuit to understand the nature of light continues even today. In the light of newer evidences, the current understanding could get modified or even replaced, for which we need to build a generation of students who can think rationally and scientifically; the seeds of which are sown in our science classes.